1. Filed of the Invention
This invention relates to a temperature sensor for a heated roll and, more particularly, to a temperature sensing device applicable to the heated roll fixing system of electrostatic copying machine.
2. Description of Prior Art
A heated roll fusing system is well known in the art as a fusing system in electrophotography which is advantageous in its thermal efficiency and low possibility of causing fire. In such a fusing system, normally, the heated fuser roll has a heater therewithin and has its surface covered with an inadhesive material such as, for example, tetrafluoroethylene resin or the like, for preventing the roll surface from adhesion of toner. Also, the pressure roll of metal has its outer peripheral surface covered with a resilient, inadhesive material such as, for example, silicon rubber or the like which prevents adhesion of the toner.
In addition, the temperature of the heated fuser roll surface is controlled within a temperature range predetermined according to feed rate of support material carrying toner images and physical properties of a toner material in order to effect desirable fixing without being involved in so called "offset" process. For this purpose, the fusing system is necessarily provided with a temperature sensing device for detecting the roll surface temperature to determine if it is getting higher or lower than the predetermined temperature range. The temperature sensing device is connected to a control system responsive to the sensed temperature for controlling the heater so as to maintain the roll surface temperature within the predetermined temperature range.
A fusing system has been proposed in the art having its temperature sensing device disposed within the heated fuser roll which is rotated during operation. This requires a complicated construction which increases costs and presents other problems, and may produce faulty copies. Also, another fusing system is well known in the art having its temperature sensing device disposed in direct contact with the surface of the heated fuser roll. However, as mentioned above, the heated fuser roll has its surface covered with a resin film which is softer than the metal sensing member and which is heated in operation to a temperature of hundreds degrees. Therefore, the sensing member is apt to scratch and injure the surface of the heated roll, causing toner images contacted with the scratched surface to be unfixed. The removal of the resinous layer caused by excessive wear will result in encountering such accidents that copying papers carrying toner images are stained or wound round the heated roll.